Plumbing 101 - Reduce main supply to 1/2"

I recently had my plumber replace the main shutoff valve inside my house. Everything seems to be working fine, but I noticed that a section of the repair goes from 3/4" down to 1/2" and then back to 3/4" to the whole house (minus one outdoor faucet).

My question is, does this have any adverse affects? The water pressure seems ok, but I suspect that it is actually less than before the repair.

The 1/2" section runs about 12 inches or less. I'm not sure what amount is acceptable. ... reasonable or worth the effort to ask the plumber to fix it...

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I don't know the numbers but I remember that a 12 inch run of a smaller size (anywhere in the line) is equivalent to a longer supply line of the bigger size. Not a big deal. So far you have not posted to say you notice a difference. It works fine.

It depends on umm... supply and demand. If the supply pressure is great enough and the demand small enough it could be fine. With the 30-50 PSI of my well water, I can average 6 GPM through a hose bib connected via 1/2" copper.

Old steel pipe despite being 3/4" could be reduced to the size of a pencil over time.

There is definitely 1/2" copper. Most of the repair consists of 3/4" copper, but one section, right before it connects back to the old galvanized, uses 1/2" (3/4 copper to 1/2 copper to 3/4 galvanized). It's definitely possible that there is 1" in there somewhere, I just assumed it was 3/4"

I'm not posting a photo because it's all behind drywall that was put up the next day.

From the responses, it sounds like it's not a big deal, I just won't be using that plumber again.

If that is indeed the way it was installed, fluid dynamics will cause the flow to increase its velocity through that short piece, so it will have minimal effect on the volume. It is when the length of the pipe increases and friction loss becomes a factor that the flow is compromise. A "reduction" like this is the principle behind "venturi plate" flow meters.

It should be a big deal. You shouldn't have a copper to galv. connection, I hope he installed some brass between the two. I'd get him out there and have him fix it and do it the right way, you're losing volume and increasing the risk of a pinhole leaks down the road with decreasing the size, not to mention the direct connect of the copper to the galv. You need 6" of brass between the two.

While its not good practice to install a section of 1/2" pipe at or around the main water line, its also not the neck in the hour glass. The OP said this was near the main shut off, which places it near the water meter.

Residential water meters are typically 1/2" to 5/8" ID and only flow 15gpm before pressure drops off sharply. Most are rated for 20 or 25, but thats just a rating. Those max ratings include very large pressure drops.

If you happen to have a very large home with a 35gpm rated meter, then you better not have any reduction in pipe size at or around the meter.